Once you enable iCloud, your iPhone will no longer sync properly (or at all) with iCal UNLESS you upgrade to OSX Lion. I tried various techniques yesterday for about an hour to see if I could get all three on the same page, but I could never get it to work. Items would appear in one or two place, but never all three. iCal won't sync at all to iCloud without Lion. This is not a secret: Apple states in its promotional materials that you need Lion to use iCloud on your Mac.
I posted this to another similar question last night, but it needs to be repeated:
This is the kind of thing that I hate, hate, hate about Apple.
Why is it that if I bought a Mac before July of this year, I must pay to upgrade my operating system in order to take advantage of iCloud? Meanwhile, a Windows Vista SP2 user, who bought a new computer in May 2009, already has everything they need on their computer to use iCloud.
Why was it possible for Apple to write a free iCloud Control Panel that was compatible with existing versions of Windows, but Mac users must buy an entirely new operating system?
It's not the price that bothers me – I've already paid for it. But Lion was such a buggy mess and the interface "improvements" so useless that I "downgraded" back to SL. Now I find that I must choose between a terrible operating system and the features in iCloud.
If Apple gave Mac users the same amount of backwards compatibility that it gives to Windows users, people with OSX 10.5 (Leopard) would be able to use iCloud. That's something to think about the next time you go to purchase a new computer.
(P.S. Something else to think about: If you bought a Mac before October 2007 (when OSX 10.5 was introduced), your computer won't run iTunes 10.5 unless you buy an upgrade. But a PC user, who purchased their computer as long ago as Sept. 2004(!), can run iTunes without an OS upgrade.)